1. Technical Field
The present invention relates to a code phase error estimating method to estimate code phase error when a reception signal is a multipath signal.
2. Related Art
GPS (global positioning system) is widely known as positioning system using positioning signals, and applied to positioning device contained in cellular phone, car navigation system and the like. According to the GPS, the current position of the positioning device is determined through positioning calculation which obtains four parameters of three-dimensional coordinates indicating a position of the positioning device and a clock error based on information about positions of plural GPS satellites and pseudo ranges between the respective GPS satellites and the positioning device.
Multipath is one of the main factors which causes errors in positioning using positioning signals. An environment where multipath occurs is referred to as a multipath environment. In a multipath environment, multipath signals produced by superimposing indirect waves such as reflection waves reflected by buildings, ground or the like, transmissive waves transmitted through obstacles, and diffraction waves diffracted through obstacles on direct waves transmitted from a signal generating source of the positioning signal (GPS satellites in case of GPS) are received. Under this environment, indirect waves appear as error signals, making it difficult to decode codes.
Various technologies for reducing the effect of multipath have been proposed. For example, a method disclosed in JP-A-2000-312163 performs correlation calculation between a reception signal and plural replica codes of C/A codes having different phases for each chip. Then, the phase range where the correlation value is expected to become the peak value is calculated. Subsequently, correlation calculation for the phase range between the reception signal and plural replica codes having different phases for each 0.1 chip is performed to detect code phase.
However, it is generally difficult to judge whether the reception signal is a multipath signal or not. Thus, a preventive method is usually employed which reduces the effect on the positioning calculation without judging whether the reception signal is multipath signal or not even when the signal is in fact a multipath signal. For example, the technology disclosed in JP-A-2000-312163 is a method for detecting appropriate code phase even when the reception signal is a multipath signal. Such preventive methods are imposing more calculation and a larger scale of the circuit.